Most computerized tomographic (CT) scanners in present use are designed to operate as either "rotate-rotate" ("third generation") scanners or "rotate-only" ("fourth generation") scanners. A "rotate-rotate" scanner is one in which both the X-ray tube and the detector array are mounted on a common rotor that is rotatable on a stationary member about a central axis. A "rotate-only" scanner is one in which only the X-ray tube is rotatable on a stationary member about a central axis. In both cases, the stationary member is provided with a central axial aperture concentric with the axis of rotation of the X-ray tube. The aperture has an axial length greater than the height of a normal person and a diameter sufficient to enable a prone patient to be moved into and out of the scanner along its central axis. As a result of this configuration in the third generation scanners, the X-ray tube is mounted on a rotatable ring that is supported on bearings carried by a stationary ring on the stationary member.
In the rotate-rotate scanner, to operate the X-ray tube, electrical power in the range of 20-60 KW must be transferred to the rotatable member and the final operating voltage in the range to 100-150 KV must be supplied to the tube. In addition data from the detector array in the rotate-rotate scanners must be transferred to processing equipment that remains stationary. Control signals have to be supplied to the rotor to control operation of the X-ray tube, among other things. Conventionally, the required power and data (including control signals) are transmitted to and from the rotatable member via flexible high voltage cables for the power and shielded cables for the data. Cable uptakes or spooling systems are provided which enable at least one complete rotation of the rotatable member to occur.
More recently, new designs have been used for transferring both data and power to and from the rotatabale member. See, for example, the Patent Application entitled "Power Transfer Apparatus Particularly for CT Scanner", which was filed in the U.S. on Jul. 18, 1988, and received Ser. No. 200,680. That Application describes a unique inductive power transfer method which enables discarding the use of the flexible cables and spooling systems for the transmission of power.
Data transmission devices are found, for example, in U.S. Pat. No. 4,794,796, which covers a system that transmits data from a rotating device to a stationary part through a rotating wave guide.
Another data transmission device is described in U.S. Pat. No. 4,259,584. There data generated by the detector of a CT scanner is transmitted to stationary processing equipment using a ring of light conducting material bent around the center of rotation of the rotatable member. A light source emits light signals corresponding to the data signals transmitted onto the surface of a ring of light conducting material. The ring conducts the light signals over its entire circumference and has a coupling location at which a light receiver is arranged on the stationary part of the scanner.
Other rotating data transmission devices specially designed for the use with CT systems are disclosed in U.S. Pat. No. 4,323,781 and U.S. Pat. No. 4,427,983 wherein the data to be transmitted from a rotating annular part to a stationary annular part is achieved by discreet pluralities of transmitters and receivers.
Optical communication links using hollow tubes have not been used because of the difficulties of constructing such tubes with proper surfaces. Proper surfaces must be highly reflective and smooth to assure that the angle of reflectance to the wall of the tube in general is equal to the angle of incidence to the wall of the tube with a minimal of light diffusion. Even if the inner periphary of the hollow tube had the proper surface, it was widely believed by those skilled in the art that the hollow tube having an arcuate shape necessary to mount the tube about the rotor would cause light ray divergence. Such divergence would, of course, increase the number of reflections and thereby attenuate the intensity of the light reaching light receivers. The intensity of light at the receiver is given by the equation: EQU I.sub.r =I.sub.o R.sup.n
where:
I.sub.o =the original intensity of the light PA1 R=reflectance, and PA1 n=number of reflections.
Since R is a fractional value, it is apparent that as the number of reflections increase the light intensity at the receiver decreases drastically. Because the divergence of light beams in light conductive material is minimized and the reflectance is relatively high, such materials though expensive, have been the material of choice for optical communication links, especially where the path is curved and the light is incoherent.
In summary then, the prior art shows the transmission of data and control signals using either light conductive material curved around the centre of rotation (for example, U.S. Pat. No. 4,259,584) or a rotatable wave guide having transmitters coupled thereto that is mounted on the rotor with receivers mounted on the stator. (See, for example, U.S. Pat. No. 4,796,183). More receivers than transmitters are used to enable utilization of more than one "channel" in the wave guide at the same time. Wave sinks are provided to divide the circular wave guide into a plurality of sub-paths. The patent explains that in case of transmitting from the stator to the rotor, then the wave guide would be mounted on the stator and along with the transmitters while the receivers would be mounted on the rotor. Hence, according to the patent, the wave guide is mounted either to the stator or to the rotor or there is a wave guide mounted to the stator and another wave guide mounted to the rotor for transmission of control and data signals, respectively to and from the stator. The prior art requires either a multiplicity of complicated wave guides or solid light transmitting material rotating with the rotor about the stator axis for the transmission and receipt of data and control signals.
Thus, the prior art does transmit control and data signals to and from the rotating part of the gantry in a manner enabling continuously rotating the gantry over many revolutions without having to reverse and return to the zero degree point as was required when cables were used for coupling the rotary part of the gantry to the stationry part of the gantry. However, the prior art used for transmitting signals to and from the rotary part of the CT gantry requires complicated and expensive microwave or solid light transmitting materials.